Input capacitors are used in certain applications to AC couple an input or an output to next stage devices. For example, sensors used in a disk drive application may need to be coupled to an input pin of a preamplifier circuit, via an input capacitor as an AC coupling capacitor. As circuit density and bandwidth requirements for many state of the art applications such as preamps for hard disk drives increase, fine line CMOS technology is used. To achieve the power, bandwidth and die size goals, the CMOS device dimensions are scaled down to sub micron dimensions. The operating voltage of these devices range from −3.0 Volts to +3.3 Volts. The voltage ratings of the devices are also scaled down. For example, in a 0.25 um CMOS technology, the devices typically are rated for voltages less than 3.6 Volts across the CMOS devices. Sometimes, this voltage is referred to as voltage stress limit or voltage stress threshold value.
In some embodiments, MOS devices are used as coupling capacitors. The MOS devices require DC bias voltage across them to work properly as capacitors. As the input voltage swings between −3.0 Volts to +3.3 Volts, DC biased MOS device based capacitors may be subjected to voltages beyond the rated voltage range for MOS devices.
The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.